This invention relates to a method of forming thermoplastic composites using electro-polymerization in a predominately aqueous solution. More particularly, this invention relates to the formation of thick and thermally stable uniform coatings of thermoplastic materials onto the surface of conductive filler materials. This invention specifically relates in a first embodiment to the formation of a copolymer of 3-carboxyphenyl maleimide and styrene as a thick coating on a suitable conductive material such as graphite fibers using electro copolymerization techniques. In a second embodiment, this invention relates to the formation of high glass transition temperature polymer coatings onto the surface of conductive filler materials (such as graphite fibers) wherein the polymer is derived from cyclic N-substituted methacrylamide monomers. This invention provides an electropolymerized polymer composite which is processible as a thermoplastic, but undergoes a crosslinking step upon heat curing so as to exhibit thermoset properties such as high strength, high glass transition and high resistance to flow and solvents.
High performance resins with a good long term environmental stability over a wide range of temperature and with damage tolerance are currently in great demand, particularly in aerospace applications. These resins are typically needed in the manufacture of composite materials which require the properties of high strength and low weight. Thermosetting resins such as epoxy systems are the most widely used matrix resins for such advanced composites. Unfortunately, they generally possess insufficient hot/wet properties and temperature resistance. An exception to this generality is bismaleimide polymers since they are stable at elevated temperature (approximately 200.degree. C.) and have good hot/wet properties. The problem with such systems is that the processing times are relatively long and there is no resin flow after the cure reaction is completed. Greater toughness and impact resistance are also highly desirable.
These problems have heightened the search for alternative thermoplastic materials for use as matrices for advanced composites. Advantageous features of thermoplastic matrices include high toughness, easy processibility, long shelf life and potential for high volume processing resulting in low cost per part. However, the difficulty in preparation of prepregs from high viscosity thermoplastic resins and the problem of wetting all the individual fibers in a fiber bundle as well as the problem associated with polymer solubility and solvent removal limit the efficacy of use of thermoplastics in advanced composites.
Electropolymerization has been used in the direct formation of polymers onto electrode surfaces such as graphite fibers. Such polymerizations have generally been from non-aqueous solutions, by ionic or mixed ionic-radical mechanism. However, molecular weights of the electropolymers from these solutions have generally been low.
More recently, aqueous solution electropolymerization techniques have been disclosed to apply thin (less than ten (10) weight % polymer) polymeric coating onto graphite fibers. See Bell et al, Polymer Composites, 8,46 (1987), Subramanian et al, Polymer Engr. Sci., 18,590 (1978). Unfortunately, such thin deposition of thermoplastic polymers onto graphite fibers does not satisfy the need for new thermoplastic composite materials since the thin coatings are incapable of forming the required thick thermoplastic matrix needed in such composites.
Experimental work is known with regard to the electropolymerization of acrylamide monomers (see M. Cvetkovskaja, T. Grcev, L. Arsov and G. Petrov, Kem. Ind. 34, 235 (1985) and J. R. MacCallum and D. H. MacKerron, The Electropolymerization of Acrylamide on Carbon Fibres, British Polymer Journal, Vol. 14, March 1982, pp. 14-18). However, the electropolymerized polyacrylamide composites described in the prior literature suffer from certain disadvantages and deficiencies including high water absorption, low temperature resistance, high solubility rates in water and thin polyacrylamide coatings on fillers such as graphite fibers.